Starting with SQL Server 2012 you cannot install on Windows XP or Windows Server 2003. So if you have these older operating systems you can only get SQL Server 2008 R2. If you want to use SQL Server 2012 then you will need at least Windows Server 2008 or Windows Vista. See Hardware and Software Requirements for Installing SQL Server 2012 for details, note that you can use the same link to see the SQL Server 2008 R2 requirements.